1. Field of the Invention
The present invention relates generally to an automatic sewing machine, and more particularly to an automatic sewing machine capable of performing a jump stitching operation in which the work fabric is fed while the needle is held at a predetermined position above the work fabric.
2. Discussion of the Prior Art
In an automatic sewing machine, the work fabric supported by a fabric holder is fed by a feed device in an X-Y coordinate system, while the needle carried by the needle bar is positioned above the work fabric, during each reciprocating movement of the needle bar which takes place for each revolution of the main spindle. Thus, a stitch having a desired length is formed for each reciprocation of the needle bar for each revolution of the main spindle. However, the feed distance of the fabric that can be obtained while the needle is above the fabric during each revolution of the main spindle is limited. To obviate this limitation, there has been proposed a needle-bar jump device which operates to hold the needle bar at a predetermined position to maintain the needle above the fabric, when the desired stitch length is larger than the maximum stitch length. This needle-bar jump device, which permits the work fabric to be fed a distance larger than the maximum stitch length, is disclosed Publications 2-4706 and 2-13061 of Japanese Patent Applications, which were laid open for opposition purpose, in Jan. 30 and Apr. 3, 1990, respectively. Automatic sewing machines are adapted such that the fabric holder is moved to feed the fabric only during a portion of the time duration of each revolution of the main spindle, which portion corresponds to the time span normally assigned to the movement of the needle above the fabric when the needle-bar jump device is not operated. In other words, the fabric holder is not moved during the other portion of the time duration of each revolution of the main spindle, which is assigned to the movement of the needle below the fabric when the needle-bar jump device is in the non-operated state.
The needle-bar jump device is operated by activation of a suitable actuator such as a solenoid. A jump stitching operation implemented as disclosed in the above-identified publications is shown in FIG. 10, which indicates intermittent activation of the feed device and the solenoid of the needle-bar jump device. Dashed line in FIG. 10 indicates the reciprocating movements of the needle when the needle-bar jump device is not operated.
The length of each stitch is represented by stitch data, whereas the maximum stitch length is determined by the maximum feed distance of the fabric holder for each revolution of the main spindle (i.e., maximum feed distance during the time when the needle is above the fabric). If the commanded stitch length represented by the stitch data is considerably larger than the maximum stitch length or maximum feed distance, the needle-bar jump device must be activated intermittently a considerably large number of times, as indicated in FIG. 10. This results in an accordingly long jump stitching operation for one jump stitch, increasing the overall sewing time and lowering the sewing efficiency.